Modern sewage works produce vast quantities of sewage sludge. Once dewatered, the sludge must be disposed of and this is usually done by dumping the sludge in the sea or in a landfill.
Whilst dewatered sludge is known to have soil improving properties, it cannot be used for this purpose without further treatment as it contains pathogens. These can be removed by heat and it is known to compost the dewatered sludge before it is dug into the soil or used for other agricultural purposes, for example, as a growing medium.
It has been proposed in the art that the dewatered sludge be mixed with an organic material such as straw, bark, wood chips, leaves etc before being composted. The function of the solid material is to create air voids in what would otherwise be an air impervious mass of dewatered sludge. A mass of sludge that air cannot enter does not compost but rots, obnoxious odours being emitted by the rotting sludge.
Whilst the beneficial properties of composted sludge are known, and there have been many proposals as to how the dewatered sludge should be treated before being composted, it is a fact that only a small percentage of sewage sludge is used for agricultural purposes.
Experimental work has shown that a characteristic of dewatered sludge is its ability to agglomerate into lumps. If dewatered sludge and a product such as wood chips are mixed by hand, for example, by using spades to turn a pile of sludge and wood chips, lumps readily form. As air can only penetrate about 1.5 mm into dewatered sludge, it will be understood that even small lumps will only be aerobic on the surface. The centre consequently rots during composing. Consequently, whilst it is known that the mixing procedure should avoid lump formation, in practice it is found that with known mixing methods, lump formation is inevitable.